Recently, global warming has been played up as environment-related issue on a mass global scale. It has been revealed that increase in concentration of carbon dioxide (CO2) in the atmosphere is one of main causes of the global warming, and thermal power plants are attracting attention as fixed sources of discharging the matter. Used as fuel for thermal power generation is petroleum, natural gas or coal. Especially coal, which has great mineable reserves, is expected to have increasing demand in the future.
Coal has much carbon content in comparison with natural gas and petroleum and includes components such as hydrogen, nitrogen, sulfur and ash as inorganic matter, so that air combustion of coal brings about combustion exhaust gas substantially composed of nitrogen (about 70%) and including others such as carbon dioxide (CO2), nitrogen oxides (NOx), sulfur oxides (SOx), steam, dust comprising ash and unburned coal particles and oxygen (about 4%). Thus, the combustion exhaust gas is discharged through a stack to the atmosphere after exhaust gas treatments such as denitration, desulfurization and dust removal are conducted to make NOx, SOx and fine particles less than environmental emission standard.
NOx in combustion exhaust gas may be thermal NOx produced through oxidation of nitrogen in air by means of oxygen and fuel NOx produced through oxidation of nitrogen in fuel. Conventionally employed for decrease in production of thermal and fuel NOx are combustion with lowered flame temperature and combustion with a NOx-reductive excessive fuel region in a combustor, respectively.
In combustion of coal or sulfur-containing fuel, the combustion exhaust gas includes SOx which is removed by a dry- or wet-type desulfurizer.
Carbon dioxide produced in larger quantity in combustion exhaust gas is also desired to be removed with higher efficiency. Conventional approach for withdrawal of carbon dioxide in combustion exhaust gas is, for example, absorption into amine or other absorbing solution, adsorption to solid adsorbent or separation through membrane. However, any of them has lower efficiency and is not in practical use.
Then, combustion of fuel by means of oxygen in lieu of air has been proposed as effective technique for separation of carbon dioxide and suppression in production of thermal NOx in combustion exhaust gas (see, for example, Reference 1).
Oxygen combustion of coal produces no thermal NOx and brings about combustion exhaust gas substantially composed of carbon dioxide and including others such as fuel NOx and SOx, which fact makes it relatively easy to cool the combustion exhaust gas for liquefaction and separation of carbon dioxide.
However, oxygen combustion involves a technical problem of higher flame temperature to be dealt with, for example, by improving heat resistance and lifespan of materials of a combustion furnace. As shown in Reference 1, known as a countermeasure for the problem is exhaust gas recirculation by branching combustion exhaust gas from a combustion furnace after exhaust gas treatment and mixing the branched combustion exhaust gas with combustion gas such as oxygen or air to be fed to the combustion furnace.
For the exhaust gas recirculation, disclosed in Reference 1 are means for cooling combustion exhaust gas treated by combustion exhaust gas treatment means to −80° C. or less for liquefaction and storage of carbon dioxide so as to separate the same from oxygen, a compressive blower for compressing the oxygen for liquefaction and storage thereof and means for vaporizing the stored oxygen for recirculation to a combustion gas supply line.
[Reference 1] Japanese Patent No. 3068888